On 3.5.2 and 3.5.3, but not 3.4.3 or 2.7.10 (Windows 64-bit), the third line causes Python to crash with exit code 0xC0000417.

import msvcrt
fp = open('file.txt', 'w')
msvcrt.locking(fp.fileno(), msvcrt.LK_NBLCK, -1)
print('End')

I came across this in the portalocker module. I don't think -1 is a valid third argument, but it doesn't raise an exception on Python 3.4 or 2.7.